This invention pertains to a process for the preparation of 2-aryl-2H-benzotriazoles and derivatives thereof. More particularly, the invention relates to an improved process for preparing 2-aryl-2H-benzotriazoles whereby high yields of the desired products are obtained and effluent pollution problems occurring with present processes for making such products are greatly reduced.
The known chemical and electrolytic reduction processes for preparing 2-aryl-benzotriazoles are not practical or economically attractive in many cases. The widely used zinc dust and sodium hydroxide system can produce effluent pollution problems in respect to waste disposal of zinc sludge which is of increasing environmental concern.
It is therefore an object of this invention to provide an improved process for the preparation of 2-aryl2H-benzotriazoles mitigating severe pollution and environmental problems.
A further object of this invention is to prepare 2-aryl-2H-benzotriazoles by reducing and cyclizing the corresponding o-nitroazobenzene under certain conditions hereinafter set forth in greater detail whereby high yields of the products can be obtained in acceptable purity.
The 2-aryl-2H-benzotriazoles have found wide use as dyestuff intermediates, optical brightener blue fluorescent agents and selective ultraviolet light absorbing stabilizers affording valuable protection for fibers, films and a variety of polymeric structures subject to deterioration by ultraviolet radiation. These materials have become important items of commerce.
The 2-aryl-2H-benzotriazoles are complex organic molecules which require careful synthetic procedures for their production in good yield and purity.
These materials can be prepared by a variety of methods, but most conveniently by either Process I, the oxidation of o-aminoazobenzene intermediates, or Process II, the reduction of o-nitroazobenzene intermediates.
Process I:
The oxidation of o-aminoazobenzene intermediates proceeds schematically as seen in Equation A ##STR1## where oxidizing agents such as sodium hypochlorite, ammoniacal copper sulfate, air in aqueous or aqueous-pyridine solution, hydrogen peroxide, hexavalent chromium compounds, potassium permanganate and the like may be used. This process is described in U.S. Pat. Nos. 2,362,988, 2,784,183, 3,055,896 and 3,072,585.
Process II:
The reduction of o-nitroazobenzene intermediates proceeds schematically as seen in Equation B where a variety of reducing agents ##STR2## may be employed as seen from the teachings of U.S. Pat. No. 2,362,988. These include alkali sulfides, zinc and ammonia at 80.degree.-100.degree. C., sodium hydrosulfide, zinc and hydrochloric acid and ammonium sulfide. The use of ammonium sulfide was also reported by S. N. Chakrabarty et al, J. Indian Chem. Soc., 5, 55 (1928); CA, 23, 836 (1929) with mixed results depending on the presence or absence of substituent groups on the 2-aryl group. In some cases the desired 2-aryl-2H-benzotriazole was not formed at all with the product of the reduction being only an aromatic amine.
Electrolytic reduction of o-nitroazobenzene intermediates was reported by H. Itomi, Mem Coll. Sci. Kyoto Imp. Univ., 12A, No. 6, 343 (1929); CA, 24, 2060 (1930) with the use of a copper cathode in dilute sodium hydroxide solution. Yields varied from 25 to 60% depending on specific embodiments and conditions, but with a major impurity being formed, namely the corresponding o-aminoazobenzene by-product.
The use of zinc dust and sodium hydroxide as the reducing system for the o-nitroazobenzene intermediates was reported by K. Elbs et al, J. Prakt Chem., 108, 209 (1924); CA, 19, 514 (1925). The yields reported varied from 30 to 85% depending on the specific o-nitroazobenzene intermediates involved.
U.S. Pat. Nos. 3,055,896 and 3,072,585 also teach the use of zinc dust and sodium hydroxide as the reducing system for o-nitroazobenzene intermediates. In these cases large molar ratios of sodium hydroxide to the o-nitroazobenzene intermediates (6-20 to 1) are taught. The yield and purity of products are not taught, but further recrystallization is indicated as necessary to obtain products of good purity.
U.S. Pat. Nos. 3,230,194 and 3,773,751 also teach the use of large molar ratios of sodium hydroxide to the o-nitroazobenzene intermediate namely 6.7 to 1 and 4.4 to 1 respectively. Recrystallization is indicated as necessary to obtain a pure product.